When WeCompress displays the message "Unsupported file feature" it usually relates to a feature enabled in Office files called 'Fast Save'.
How Fast Save Works
Office 2003 and earlier have the option to enable (or disable) 'Fast saves'. When Fast Save is enabled, the act of saving a file doesn’t actually involve your entire file when you click the save button located on the toolbar, or when you press the CTRL+S hotkey. Instead, it only appends the changes you have made to the original document. In this way, the amount of information being saved with each save command is significantly reduced.
Resolution: Enable WeCompress to compress the file
There are two ways to resolve the issue before resubmitting to WeCompress to compress: